Answer:
If you wanted to write an equation it would be:
12 + 5h = g
-h representing the hours
-g representing gallons of water
(you don't have to use those variables, just for a reference)
So if you want to know how many gallons there would be after a certain number of hours, just plug the number in where h is and solve.
Example:
10 hours
12 + 5(10) = g
12 + 50 = g
g = 62
62 gallons of water after 10 hours